1. Field of the Invention
The present invention relates to a spread illuminating apparatus used as an illuminating means for signboards, various kinds of reflection-type display devices, and more particularly for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device operated with small power consumption is characterized by low profile and light weight, and the demand mainly for display devices in cellular phones and personal computers has been increased. Since a liquid crystal, which is a component of the liquid crystal display device, does not emit light by itself unlike a light emitting element such as a cathode-ray tube, an illuminating means is required for viewing an image. In particular, in the recent demand for lower profile, there is a growing tendency to employ a thin plate-like spread illuminating apparatus of side light type (light conductive plate type) as an illuminating means for the liquid crystal display device.
An example of such a side light type spread illuminating apparatus is disclosed in Japanese Patent Application No. 2000-170761 by the applicant of the present invention.
As shown in FIG. 6, this spread illuminating apparatus 1 is generally composed of a transparent substrate (guide plate) 2 made of a light-transmissible material and a bar-like light source 3 disposed close to an end surface 2a of the transparent substrate 2. Light rays emitted from the light source 3 are introduced into the transparent substrate 2 and irradiate a liquid crystal display device (not shown) disposed under the transparent substrate 2, whereby the spread illuminating apparatus illuminates auxiliarily the liquid crystal display device.
The light source 3 is generally composed of a light conductive member (guide rod) 4 made of a transparent material, formed in a rod-shape and disposed close to and along the end surface 2a of the transparent substrate 2, and spot-like light sources 5 and 6, for example LEDs, disposed on one end 4a and the other end 4b of the light conductive member 4, respectively.
An optical path conversion means 7 is provided on a surface 4d of the light conductive member 4 opposite to a surface 4c facing the transparent substrate 2. The optical path conversion means 7 comprises a large number of grooves 7a shaped substantially triangular in section and arrayed in the longitudinal direction, and is adapted to uniformly spread light rays emitted from the spot-like light sources 5 and 6 to the surface 4c of the light conductive member 4.
A light reflection pattern 8 composed of a large number of grooves 8a shaped triangular in section and a large number of flat portions 8b each present between adjacent grooves 8a is formed on one surface (an upper side in FIG. 6, hereinafter referred to as “upper surface” 2c) of the transparent substrate 2. The large number of grooves 8a each extend running parallel to the light conductive member 4, and are arrayed from the end surface 2a to an end surface 2b opposite to the end surface 2a. The ratio of the width of the grooves 8a to the width of the flat portions 8b gradually increases in proportion to the increase in distance from the light conductive member 4, whereby light rays coming in from the light conductive member 4 are reflected almost uniformly everywhere at the transparent substrate 2 irrespective of the distance from the light conductive member 4, and irradiate the liquid crystal display device (not shown) disposed under the transparent substrate 2.
In the above-described spread illuminating apparatus 1, since the transparent substrate 2 does not have its upper surface 2c covered by anything when combined with other members, the upper surface 2c can be damaged and broken pieces of the transparent substrate 2 or other refuse can get in the grooves 8a of the light reflection pattern 8 in assembly process or during use, whereby the light conductivity of the transparent substrate 2 may be degraded.
To solve the above-described problem, the applicant of the present invention proposed a spread illuminating apparatus in Japanese Patent Application No. 2000-170761 in which a film is provided close to an upper surface of a transparent substrate so as to cover a light reflection pattern formed on the transparent substrate.
In this spread illuminating apparatus, the film prevents the refuse from getting on the light reflection pattern during the assembly process, thereby protecting the surface of the transparent substrate against damages. However, the film is easily brought into direct contact with the transparent substrate, which can damage the light reflection pattern, and as a result the light conductivity may be degraded.